1. Field of the Invention
The present invention relates chiefly to a surface light source device to be used in backlight units of liquid crystal display panels or the like.
2. Description of the Related Art
In recent years, liquid crystal display devices have been rapidly expanding in application range by virtue of their features such as light weight, thin thickness and low power consumption drive. Liquid crystal display devices, which are not self light-emitting devices, generally need a separate light source such as a backlight. A backlight unit is composed mainly of a light source and a light guide plate for guiding light outputted from the light source to provide surface emission.
Generally, backlight units can be classified into direct type, edge light type, or the like depending on the position of the light source. With a direct type backlight unit, a multiplicity of light sources, e.g. cold-cathode tubes or LEDs (Light Emitting Diodes), are provided directly under the light guide plate so that light incoming from the light sources is diffused at the light guide plate so as to be applied to a liquid crystal display panel via a plurality of optical sheets. With an edge-light type backlight unit, a light source is provided at a side end of a light guide plate so that light incident from the light source is diffused at the light guide plate so as to be applied to a liquid crystal display panel via a plurality of optical sheets.
FIG. 28 shows an example of an edge-light type backlight unit in which LEDs are adopted.
As shown in FIG. 28, a plurality of LEDs 106 serving as point light sources are placed in a side face of the light guide plate 101. Also, a diffusion sheet 102 is placed above the light guide plate 101, and the diffusion sheet 102 widely diffuses light outputted from the light guide plate 101. Further, a prism sheet 103 is placed above the diffusion sheet 102, a prism sheet 104 is placed above the prism sheet 103, and an optical sheet 105 is placed above the prism sheet 104, where the individual sheets act to converge light into visual directions so that higher luminance can be achieved.
For material of the light guide plate 101, transparent resin panels or the like are used conventionally. The light guide plate 101 has an incident surface at a side face between an outgoing surface and a bottom face opposed to the outgoing surface, the incident surface having a primary light source such as a cold-cathode tube or point light sources given by a plurality of arrayed LEDs 106. Also, scattered dots are printed on the bottom face of the light guide plate 101, so that luminance distributions in the visual directions are controlled so as to be uniformized by adjusting size, density or the like of the dots. With such an arrangement, light outputted from the primary light source becomes incident on the light guide plate 101 via the incident surface, passing through inside of the light guide plate and outputted from the outgoing surface toward the liquid crystal display part. However, due to an influence of the scattered dots in the bottom face of the light guide plate, light immediately after being outputted from the light guide plate 101 has an orientation distribution spreading to a wide angle, making it difficult to direct the light toward the visual directions.
Therefore, two prism sheets 103, 104 parallel to each other as well as an optical sheet 105 are used in order to converge the light outputted from the light guide plate 101 to the visual directions for higher luminance. However, with such an arrangement, the prism sheet, which is high priced, needs to be used two in number, causing an increase in the number of component parts of the unit so that the assembly becomes complicated as a problem.
Accordingly, in order to solve the above problems, there have been made many proposals for directing the light outputted from the outgoing surface toward the visual directions by forming prisms in the outgoing surface of the light guide plate or the bottom face opposed to the outgoing surface or the like. However, placement of prism in regular array causes hot spots (light-gathering portions, luminous spots) or bright/dark lines to occur in vicinities of the incident surface, with the result that the uniformity of the surface light source is impaired. Also, the placement interval of LEDs has been increasing, as compared with conventional ones, together with increasing brightness per LED chip, resulting in more influences of the directivity of LEDs.
For solution of the above problems, there has been made a proposal for improving the hot spots and bright/dark lines by using a plurality of prisms in the shape of the outgoing surface (see, e.g., Document 1).
FIGS. 29 and 30 show an outgoing surface shape of a light guide plate 22 described in Document 1.
FIG. 29 is a perspective view of the light guide plate 22 in Embodiment 1 of Document 1. The light guide plate 22 has an incident surface 221 for a light source, and an outgoing surface 223 adjacent to the incident surface 221. The outgoing surface 223 is composed of a first region 223A adjacent to the incident surface 221, and a second region 223B adjacent to the first region 223A at a position different from that for the incident surface 221.
FIG. 30 is a top view of the light guide plate 22. A boundary between the first region 223A and the second region 223B is assumed as an imaginary borderline parallel to the incident surface 221.
As shown in FIGS. 29 and 30, in the second region 223B of the outgoing surface 223, a plurality of elongate prism lenses 225 are formed, where each of side edges of the prism lenses 225 extends along a direction orthogonal to the incident surface 221. Further, in the first region 223A of the outgoing surface 223, a plurality of elongate tetrahedral lenses 226 are formed in regular and periodical placement.
As shown in FIG. 29, the prism lenses 225, each made from a V-shaped protrusion, are placed in array all over the second region 223B. FIG. 31 shows a cross-sectional view taken along the line III-III of FIG. 30, and each V-shaped protrusion has a triangular cross section as shown in FIG. 31.
As shown in FIG. 31, according to Embodiment 1 of Document 1, an apex angle θ1 of each triangular cross section is set to about 175° or less. A length of a lower end of each triangular cross section is set to about 2.0 mm or less, and a height H of each triangular cross section of the individual prism lenses 225 is also set to about 2.0 mm or less.
Referring to FIG. 29, the tetrahedral lenses 226 are arrayed in correspondence to the prism lenses 225 along a direction parallel to the incident surface. Each tetrahedral lens 226 has a first side face 2261 facing a prism lens 225, mutually opposed second side face 2263 and third side face 2265, and a bottom face (not shown). The first side face 2261 is located at the borderline III-III of FIG. 30. Also as shown in FIG. 30, the first side face 2261 of the tetrahedral lenses 226 and a terminal end of the prism lenses 225 are interconnected with each other, respectively, at the borderline III-III,so that the tetrahedral lenses 226 and the prism lenses 225 are connected to each other, correspondingly and respectively. As a result, in correspondence to the apex angle θ1 of each triangular cross section of the prism lenses 225, the apex angle of each first side face 2261 is also set to about 175° or less. Besides, the length of the lower end of each first side face 2261 is set to about 2.0 mm or less, and the height H of each first side face 2261 is also set to 2.0 mm or less.
FIG. 32 is a cross section taken along the line IV-IV of FIG. 30. As shown in FIG. 32, a ridge defined by the second side face 2263 and the third side face 2265 of each tetrahedral lens 226 has a projected angle θ2 and an adding point relative to the outgoing surface 223 (a point at which the incident surface 221 and the ridge line of the second side face 2263 and the third side face 2265 intersect each other). According to Embodiment 1 of Document 1, the angle θ2 is set to about 85° or less.
Document 1 proposes that adopting the above-described structure makes it possible to suppress hot spots or bright/dark lines occurring in the outgoing surface.
Document 1: U.S. Pat. No. 7,431,491
However, with the above-described structure of Document 1, since each prism is made up from regions of the outgoing surface adjacent to the incident surface, widened placement intervals of LEDs would cause bright/dark portions to occur in portions of the outgoing surface in vicinities of the incident surface due to influences of the directivity of the LEDs. Besides, setting outer dimensions of the light guide plate equal to the effective light-emission area would cause the light guide plate to be expanded and contracted by influences of heat of the LEDs, so that it may become impossible to output light stably from the effective light-emission area. Therefore, it is desirable that the outer dimensions of the light guide plate be larger than the effective light-emission area. In such a case, with the prism shapes present outside the effective light-emission area, light that is originally not intended for use in the liquid crystal display device would be outputted to the outgoing surface in the prism shape regions. As a result, use efficiency of light would lower, involving more than necessary power consumption.
Also, with the structure of FIG. 30, light spreading in portions of the outgoing surface in vicinities of the incident surface becomes larger, so that scrolling light emission of the liquid crystal screen causes light to leak to unnecessary places. Such light is seen as an afterimage in the screen so as to deteriorate the screen quality, leading to an impairment of the split light-emission property.